Process of making liquid electrostatic developers containing gelatin

ABSTRACT

A liquid developer containing a gelatin toner for developing electrostatic latent images is made by cooling an aqueous solution of gelatin to produce a gelatin gel, adding the gelatin gel to a solvent which is miscible with water but is incapable of dissolving the gelatin and mechanically pulverizing to form a dispersion of gelatin. The finely divided gel powder is collected and added to an insulating carrier liquid.

United States Patent [191 Tamai 1 PROCESS OF MAKING LIQUID ELECTROSTATICDEVELOPERS CONTAINING GELATIN [75] Inventor: Yasuo Tamai, Asaka, Japan[73] Assignee: Xerox Corporation, Stamford, Conn.

[22] Filed: Feb. 3, 1970 [21] Appl. No.:'8,4l4

[30] Foreign Application Priority Data Feb. 4, 1969 Japan 44/8345 [52]US. Cl 252/62.l, 106/125, 106/135, 260/118 [51] Int. Cl G038 9/04 [58]Field of Search 252/621; 106/125,

[56] References Cited UNITED STATES PATENTS 3,137,630 6/1964 Hecker eta1. 260/117 [451 Aug.28, 1973 2,965,484 12/1960 Tulgin et a1 96/982,527,268 10/1950 Hart et a1 96/97 2,490,749 12/1949 Fierke et a1 96/982,297,691 10/1942 Carlson 96/] 1,854,061 4/1932 Pigache 106/1352,105,413 1/1939 Dewsbury et 31,... 106/135 2,197,843 4/1940 Teewan106/135 Primary Examiner-Norman G. Torchin Assistant Examiner-J. P.Branner Attorney-James J. Ralal'iate, Albert A. Mahassel and Samuel E.Mott [57] ABSTRACT 4 Claims, No Drawings '1 PROCESS OF MAKING LIQUIDELECTROSTATIC DEVELOPERS CONTAINING GELATIN BACKGROUND OF THE INVENTIONThis invention relates to imaging materials and more particularly toliquid developers for developing electrostatic latent images such as areobtained inelectrophotography and electrostatic recording process. Morespecifically, this invention relates to improvements in the methods ofmaking liquid developers having incorporated therein a gelatin tonerwhich developer is to be used in a reproduction process which convertsan electrostatic latent image into a gelatin image which maysubsequently absorb a colored dye and transfer the dye from the gelatinmatrix to a receiver sheet.

The most popular process for obtaining multicolor prints of goodquality, is the technique which combines the chromophoric developingmethod with a silver salt emulsion. The tanning developing method whichsimilarly used silver salt emulsion and the so-called dye transfermethod or dyeimbibition method andwhich is based on the formation of agelatin'relief image-hasalso been popularized such as in technicolorprocess.

Comparison of the two methods reveals that the former method is suitablefor volume processing in a wide range of applications but isunsatisfactory in that the image has insufficient durability againstlight irradicas tion and therefore has poor storability. In contrastthereto, the dye transfer method can provide images with outstandinglight resistance and storability. This dye transfer method is similar toprinting techniques and possesses improved economy when many duplicatesare obtained from the original. When the number of duplicates to beobtained is small, however, this method is rather expensive since greattime, effort and skill are consumed in the preparation of the gelatinmatrix which serves as the plate.

Recently, a simplified method of rapidly, easily and inexpensivelyproducing a master for use in' the dye transfer process has beeninvented. United States copending application Ser. No. 780,425 filedDec. 2, 1968, discloses this novel color reproduction process, thedisclosure of which is hereby incorporated by refer ence. Thereindisclosed is a technique of producing multicolored prints of excellentquality in color and which are light fast and which comprises thefollowing steps:

1 An electrostatic latent image is formed on the photoconductiveinsulating layer of an electrophotographic recording member or on theinsulating coating of an electrostatic recording member.

2. The electrostatic latent image is converted into a visible tonerimage by using the finely divided powder of a suitable substance such asgelatin toner, for example, which readily absorbs an aqueous solution ofa water-soluble dye. l

3. The toner image is fixed by a suitable method.

4. An aqueous solution of a water-soluble dye is brought into contactwith the toner image to allow the dye to be absorbed by the image.

5. A separately prepared material having a surface capable of absorbingthe dye solution is so positioned that the dye receiving layer will comeinto contact with the tonerimage having the absorbed dye. The dyetransfers itself to the dye receiving layer producing a final image onthe dye receiving layer.

In this process, the electrostatic latent image may be developed with aliquid developer containing a pow-. dere'd toner image. Particularlydesirable results are obtained with a liquid developer containing agelatin toner. The quality of the toner image and the color printsproduced therefrom is dependent to a large extent upon the quality ofthe gelatin used and the mannerof making the gelatin. Difficulties havebeen encountered with the use of gelatin obtained from known processes.7

One process produces gelatin powder by the dry pulverization of gelatingrains. However, since grains and flakes of gelatin are solid, hard andtenacious, it is very difficult to reduce them to a uniform fine size. Asecondmethod includes the use of an aqueous solution of gelatin which isadded to and dispersed in a solvent in which gelatin is insoluble. Fineparticles of gelatin are produced and collected from the resultantdispersion. This method, however, suffers from a shortcoming that in thesteps up to the collection of the aqueous solution of gelatin, gelatintends to coagulate. Still another available method by which the gelatinparticles may be obtained is by spraying the aqueous solution of gelatininto dried air. With this method, however, it is not possible to producegelatin particles fine enough to serve as the toner in a liquiddeveloper capable of producing fine images of high resolution.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a developing system which overcomes the above noteddeficiencies.

It is another object of the invention to provide a liquid developerwhich overcomes the above noted deficiencies.

' It is another object of the invention to provide an improvedmanufacturing process for a liquid developer containing a gelatin toner.

It is another object of the invention to provide a liquid developeruseful in the preparation of a gelatin matrix to produce high qualitycolor prints by the dye transfer method.

It is another object of the invention to provide a novel method -ofproducing uniformly finely divided gelatin powder.

The above aforementioned objects and others are ac complished generallyspeaking by providing a novel process for manufacturing a liquiddeveloper containing a gelatin toner. More specifically, inmanufacturing a'liquid developer to be used for developing electrostaticlatent images according to this invention, the following steps arefollowed:

1. An aqueous solution of gelatin is cooled and soliditied to produce agelatin gel;

2. The gelatin gel is added to an organic solvent,

which is miscible with water and which is incapable of dissolving thegelatin, and the mixture is mechanicially pulverized to form adispersion of gelatin, and the finely divided gelatin powder iscollected from the gelatin dispersion; and 3. The finely divided gelatinpowderobtained is dispersed in a carrier liquid having an electricresistance greater than about l0 ohm-cm.

Better imaging results are obtained when the additional step of washingthe resultant finely divided gelatin powder with an organic solvent,which is miscible with waterand which is incapable of dissolvinggelatin,

is accomplished between steps (2) and (3). The washing step serves toeliminate the moisture contained in the finely divided gelatin powder.

Any suitable gelatin may be used in the present invention. Typically,the gelatin may be selected from either skin gelatin or bone gelatin. Itis however preferred that the gelatin not have a very wide range ofsolidifying temperatures. Ordinary refined gelatin of photographic gradeis therefore suitable for the objects of this invention.

Any suitable amount of gelatin may be dissolved in water to form theaqueous solution. When the aqueous solution of gelatin is prepared byusing gelatin of the above described nature, the gelatin concentrationis in the range of from about 1 to about 30 percent by weight of thesolution. A gelatin concentration which is too low is disadvantageousbecause the solidifying point is excessively lowered and at the sametime an excessively large volume of water is allowed to remain in thefinely divided gelatin powder obtained at the end of step (2). Inaddition, even if the above described washing step is employed toeliminate the moisture contained in the finely divided gelatin powder,the efficiency of such a washing step is low. On the other hand, if thegelatin concentration is too high, the gelatin gel becomes more solid sothat it may become difficult to obtain sufficiently fine gelatin powderby mechanical pulverization. The present invention is characterized byutilizing the fact that a gelatin gel of low concentration has anextremely low degree of mechanical strength.

The aqueous solution of geletain turns into a gelatin gel when it iscooled to the neighborhood of about C. Cooling the solution to atemperature below about -5C. for a long time should be avoided since atsuch temperatures the moisture present in the gelatin gel is frozen, andas a consequence, the homogeneous gelatin gel is spoiled.

The gelatin gel thus obtained may be cut to pieces of any suitable size.The gelatin gel may, for example, be cut into cubes each having l-cmsides, and preserved in a refrigerator, if required. As needed, thegelatin gel may be added to an organic solvent, which is miscible withwater but is incapable of dissolving the gelatin.

Any suitable organic solvent may be employed. Particularly satisfactoryresults are obtained with alcohols such as methanol and ethanol, andketones such as acetone and methylethyl ketone. The organic solvents areadded in amounts of one to I00 parts by volume per one part of gelatingel. If an insufficient volume of organic solvent is employed, it is notpossible to obtain sufficiently fine gelatin particles. If an excessivevolume of organic solvent is used, the operational efficiency isdegraded and it becomes difficult to collect gelatin in step 2 mentionedabove.

When the gelatin gel is pulverized within the above described organicsolvent, the gel is finely cut and at the same time the moisturetransfers itself from within the gel particles into the organic solvent.As a consequence of this pulverization, the gelatin gel is finelydivided and dispersed in the organic solvent. Any suitable means forpulverizing the gelatin gel may be employed. Typical pulverizing meanswhich may be employed include: a ball mill, a vibrating mill, ahomogenizer, an

chinery Company, Ltd., and the like.

The pasty dispersion which is obtained through such pulverization iscomposed of gelatin and the organic solvent contains a considerablevolume of the moisture which was contained in the gelatin gel. In thepasty dispersion, this moisture is mixed partly in the organic solventand partly in the gelatin particles. Since the presence of moisture inthe liquid developer for electrophotography is not desired, it thereforeis necessary to remove the moisture from within the finely dividedgelatin powder. For this purpose, the fine particles of gelatin presentin the paste obtained by the mechanical pulverization of the step (2)are additionally washed with any suitable organic solvent which ismiscible with water but incapable of dissolving the gelatin. Typically,the organic solvent is also selected from the group of organic solventsdescribed above. To be more specific, the gelatin particles in the pasteare separated from the paste by any suitable means such as filtration,centrifugal separation, and flocculation and subsequently redispersed inthe freshly supplied organic solvent. As a result of this treatment,which may be repeated as required, the quantity of moisture contained inthe gelatin particles is substantially decreased. Simultaneously, thegelatin particles shrink so that it may be more difficult to collect theparticles.-

The gelatin particles which have been sufficiently dehydrated by washingare recollected by any suitable means such as by filtration, centrifugalseparation, flocculation and the like and then are dispersed in thedeveloper carrier liquid. Alternatively, the gelatin particles may bedried to a powdery state and then added to the developer carrier liquid.It has been found, however, that a more stable dispersion of gelatin canbe obtained by adding the gelatin particles while still wet with theorganic solvent to the developer carrier liquid. To this end, the pasteof gelatin particles in the carrier liquid may be added to the developercarrier liquid.

Any suitable carrier liquid may be employed in the practice of thisinvention. Typically, the carrier liquid is substantially the same asthat which is used in conventional liquid developers inelectrophotography and is a nonpolar organic solvent having highelectric resistance. Preferably, the electric resistance is greater thanabout 10 ohm-cm since with a low electric resistance, destruction of theelectrostatic latent image formed on the imaging surface may occur.Examples of typical solvents which may be used as the carrier liquid arecyclohexane, kerosene, gasoline, isooctane, heptane, hexane, andchlorofluorinated hydrocarbons.

The carrier liquid may also contain various soluble components, such asnonionic surface active agents, dispersing agents, and fixing agents.Any suitable amount of gelatin may be added to the carrier liquid toform the liquid developer. The toner concentration of gelatin in thecarrier liquid is preferably within the range of from about 0.001 toabout 5 percent by weight based on the weight of the carrier liquid.When the quantity of gelatin particles within the carrier liquid isextremely small, it is impossible to sufficiently develop theelectrostatic latent image on the sensitive layer. On the other hand, ifthe gelatin toner concentration is too high, fogging tends to occur andthe dispersion stability required for the liquid developer maydeteriorate.

By using the liquid developer containing the gelatin according to thisinvention, a gelatin matrix can be prepared according to the followingsteps:

1. An electrostatic latent image is formed on a suitable imaging surfacesuch as a photoconductive insulating layer of an electrophotographicrecording member or an insulating coating of an electrostatic recordingmember.

2. The electrostatic latent image is developed by using a liquiddeveloper containing the gelatin toner.

3. The gelatin toner image thus obtained is fixed by any suitable methodto form the gelatin matrix of the printing master.

To form a color print by the dye transfer method two additional stepsare performed.

4. An aqueous solution of a water soluble dye is toner image. At thispoint, the dye transfers to the dye receiving layer to form a finalimage on the dye receiving layer.

A multiplicity of prints each carrying the dye image can be obtained byrepeating steps (4) and (5) with the same gelatin matrix.

Any suitable imaging surface may be employed. Typically, the imagingsurface comprises an insulating recording layer capable of retaining anelectric charge. It may, for example, be selected from the well knowngroup of dielectric materials and an electrophotographic sensitive layercontaining a photoconductive substance and an insulating resin on awaterproof base. In ordinary electrophotographic sensitive materials, alow electric resistant layer is provided between the sensitive layer andthe base. The well known photoconductive materials may typically beemployed.

When an electrophotographic sensitive layer using zinc oxide is employedas the imaging surface, the electrostatic latent image is generally ofnegative polarity. Since the gelatin toner according to the presentinvention assumes a positive polarity, a positive image is obtained withthe gelatin particles being attracted to the charged areas of theimaging surface. Where a selenium deposited layer is used as the imagingsurface, the electrostatic latent image generally assumes a positivepolarity. When the liquid developer agent of this invention is used onthis layer, therefore, a negative image may be obtained with the gelatinparticles being repelled from the charged areas of the imaging surface.

After the toner image is formed, it is fixed to the imaging surface inany suitable manner. Typical satisfactory methods for fixing the tonerimage to the imaging surface include thermo fixation wherein the gelatinimage is softened by the application of heat; solvent fixation wherein acertain solvent softens the toner image; and, steam fixation wherein theapplication of steam softens or melts and fixes the toner. The tonerimage may also be fixed by either embedding the image in the surface ofthe recording layer or by overcoating the image with a suitable resinousmaterial which permits the dye solution to diffuse through a thin filminto the toner material.

DESCRIPTION OF-PREFERRED EMBODIMENTS The following preferred examplesfurther define, describe and compare preferred materials, methods andtechniques of the present, invention. in the examples, all parts andpercentages are by weight unless otherwise specified. I

Five grams of photographic grade gelatin (bonegelatin) are added tomilliliters of distilled water. After standing for 30 minutes, thegelatin swelled. The gelatin is dissolved by heating themi-xture to 60C.while under agitation. The aqueous solution containing 5 percent byweight of gelatin so. obtained is placed in-a refrigerator kept at 5C.andthe gelatin is permitted to solidify. Ten (10) grams ofsolidifiedgelatin and milliliters of acetone are placed in a ball mill andpulverized forten hours. An acetone dispersion of gelatin having a milkywhite color is obtained. The gelatin particles are collected bysubjecting the acetone dispersion to centrifugal separation. Thecollected gelatin particles are combined with 100 milliliters of freshacetone and pulverized in a ball mill for one hour. The resultantgelatin dispersion is again subjected to centrifugal separation tocollect the gelatin particles. Thegeletin particles are washed anadditional time with acetone. Finally, 5 milliliters of acetone pastecontaining gelatin particles are obtained. 1

This paste is combined with 5 milliliters of soybean oil and agitatedsufficiently. The mixture is dispersed in a carrier liquid having thefollowing composition, while under the application of ultrasonic waves.

A liquid developer having a milky white color is obtained. The soybeanoil in the carrier liquidserves as the dispersant. The rosin-modifiedphenolformaldehyde functions as the fixing agent. In' other words, therosin-modified phenolformaldehyde resin remains dissolved in the carrierliquid, but, when placed in a washing'bath for example of Isopar E, anisoparaffin bath for cleaning, available from Humble Oil and RefiningCompany, it is insolubilized. The insolubilization enables the gelatintoner to be fixed.

The kerosene is added for the purpose of suitably controlling the speedat which the liquid developer is evaporated. In this liquid developer,the gelatin toner assumes positive electric charge.

Separately, 100 parts by weight of photoconductive zinc oxide and 20parts by weight of the epoxy ester of dehydrated castor oil fatty acidare mixed with a suitable quantity of toluene and mixed into a uniformcoating solution. In a small quantity of ethylene glycol monomethlyether, 1000 parts of fluorescein 20 and 100 parts of tetrobromophenylblue 20 are dissolved, and the resultant solution is added to'thecoating solution to extend the photosensitivity of the zinc oxide to theentire visible zone. The mixture, with a suitable quantity of toluenecombined therewith, is spread on a polyethylene-terephthalate filmhaving a thickness of about 90 microns and having deposited therein adry aluminum coating of about 8 microns. Whensufficiently dried in adark place, the product functions satisfactorily as anelectrophotographic sensitive material.

This electrophotographic sensitive material is exposed to a negativecorona discharge in a dark place to have its surface uniformlyelectrically charged. A color slide chosen as original is loaded on anenlarger, with a red filter laid over the slide. The negatively chargedsensitive material is exposed to light projected through the original.

The exposed sensitive material is first wetted with kerosene and then,with minimum loss of time, soaked in the above described liquiddeveloper contained in a stainless steel vat so that the vat mayfunction as the developing electrode while the surface containing theelectrostatic latent image is brought close to the vat bottom. Afterbeing submersed in the developer for about 90 seconds, the sensitivematerial is removed, washed with isoparaffm, and then dried. Afterdevelopment, the sheet is soaked in 1 percent methanol solution offormalin. The sheet is then left to stand overnight at room temperatureto allow the gelatin image to form into a hard membrane.

After the hardening treatment, the sensitive material now carrying thegelatin toner image is soaked in an aqueous solution containing 40percent by weight of acetic acid for 2 minutes. This treatment causedsubstantially all of the zinc oxide present in the sensitive layer to beremoved from the layer.

The procedure to this point produces a matrix to be used for cyanprinting.

By following the same procedure, another sheet is exposed to lightprojected through the combination of the same original with a greenfilter. With the same development technique with the liquid developercontaining a gelatin toner a matrix for magenta printing is obtained. Bycombining the original with a blue filter in the similar manner, amatrix for yellow printing is obtained.

The three matrixes are soaked for two minutes in the aqueous solutionsof Color Index Acid Blue 54, Acid Violet 7, and Acid Yellow 23respectively, removed from the baths, and then washed in a bath ofacetic acid.

A sheet having a gelatin layer thereon is soaked in aluminum sulfatesolution to be mordanted, and thereafter registered correctly with andpressed against the three matrixes, one after another. Through thistreatment, the dyes absorbed in the toner images are transferred ontothe gelatin layer. A duplicate of extremely high quality is obtained.One hundred duplicates are made using these matrixes with substantiallyno alteration in print quality.

EXAMPLE II Methylethyl ketone is used in the place of acetone as thesolvent when the gelatin gel is pulverized in the procedure of ExampleI. The other operating conditions are the same as in Example I. A liquiddeveloper having a milky white color is obtained. The gelatin toner inthis liquid developer assumes a positive electric charge. By the sameprocedure as in Example I gelatin matrixes are obtained.

EXAMPLE III Ethanol is used in the place of acetone as the solvent whilethe gelatin gel is pulverized in the procedure of Example I. The otheroperating conditions are the same as in Example I. A liquid developercontaining the gelatin toner and having positive electric charge isobtained. Through the same procedure as in Example I, gelatin matrixesare produced.

EXAMPLE'IV Methylethyl ketone is used as the solvent for washing thegelatin particles in the procedure of Example I. Gelatin matrixes areobtained by following the same procedure as in Example I.

EXAMPLE V A liquid developer is prepared by following the procedure ofExample I, while using methylethyl ketone as the solvent during the timeof pulverizing the gelatin gel. A 111 methylisobutyl ketone-acetonemixture as the solvent for washing the gelatin particles is employed.Gelatin matrixes prepared by following the same procedure as in ExampleI are obtained.

EXAMPLE VI In the procedure of Example I, methylethyl ketone is used asthe solvent during the time of pulverizing the gelatin gel. Methanol isused for washing gelatin particles, and finally acetone is used toprepare the paste of gelatin. The paste is dispersed in the same carrierliquid as in Example I and the procedure of Example I is followed withthe resultant liquid developer to produce gelatin matrixes.

EXAMPLE VII On an aluminum plate, selenium is vacuum deposited to athickness of about 60 microns to produce a xerographic plate. The plateis exposed to positive corona discharge in a dark plate to uniformlycharge the surface to (i 350V. A colored negative film is selected as anoriginal and loaded on an enlarger, with a-red filter laid on the slide.The positively charged sensitive plate is then exposed to lightprojected through the original.

The exposed xerographic plate is then wetted with kerosene and soaked inthe liquid developer described in Example I. At this time, a separatelyprepared metal plate is positioned close to the surface of thexerographic plate so as to serve as the developing electrode. The spacebetween the xerographic plate and the developing electrode is about0.2mm. In order to effect the so-called solid area development, a biasvoltage of lV was applied to the xerographic plate. After about secondsof submersion, the plate is removed and washed with Isopar E(isoparafi'in solvent available from Humble Oil and Refining Company)and dried.

Subsequently, the developed xerographic plate is soaked in a methanolsolution containing 1 percent by weight of formalin and thereafter leftto stand at room temperature for 5 hours.

A matrix for cyan printing is produced. In entirely the same manner,another xerographic plate is exposed to light projected through the sameoriginal combined with a green filter. The same treatment is giventhereafter to obtain a matrix for magenta printing. A matrix for yellowpringing is obtained by combining the same original with a blue filter.

The three matrixes are dyed respectively in the same manner as inExample I.

A sheet having a gelatin layer deposited on the surface thereof ismordanted. Thereafter, the three matrixes were registered with andpressed against the sheet, one after another. This treatment causes thedyes absorbed on the toner images to be transferred to the gelatinlayer. Consequently, a colored positive image having excellent qualitywas obtained. Forty duplicates are made using these matrixes with noappreciable change in print quality.

The xerographic plate which had been used is washed sufficiently withwarm water followed by washing with an acetone-toluene mixture andfinally washed with toluene and dried. After this treatment, thexerographic plate may be reused.

Although specific materials and operational techniques are set forth inthe above exemplary embodiments using the developing materials andtechniques of this invention, these are merely intended as illustrationsof the present invention. There are other materials and techniques thanthose listed above which may be substituted with similar results. Othermodifications of the present invention will occur to those skilled inthe art upon a reading of the present disclosure which modifications areintended to be included within the scope of this invention.

What is claimed is:

l. A method of making a liquid developer containing gelatin tonercomprising the steps of:

a. forming an aqueous solution consisting essentially of from about 1 toabout 30 percent by weight of gelatin based on the weight of thesolution;

b. cooling the aqueous gelatin solution to solidify the gelatin to forma gelatin gel;

0. adding the gelatin gel to an organic solvent which is miscible withwater and is incapable of dissolving gelatin;

d. mechanically pulverizing the gelatin gel in said solvent to form afine dispersion of gelatin in said solvent;

e. collecting finely divided gelatin powder from the gelatin dispersionand washing said gelatin powder with an organic solvent which ismiscible with water and is incapable of dissolving gelatin, so as to atleast substantially reduce the water content of said gelatin powder;and,

f. dispersing the finely divided gelatin powder in a carrierliquidvhaving an electrical resistivity greater than about 10 ohm-cm toform a liquid developer.

2. The method of Claim 1 wherein the aqueous solution of gelatin iscooled to a temperature between about 0C. and about SC.

3. The method of claim 1 wherein the gelatin powder is of substantiallyuniform size and is substantially free of water.

4. The method of claim 1 wherein the finely divided gelatin powder isadded to the carrier liquid in an amount of form about 0.001 to about 5percent by weight of the carrier liquid.

2. The method of Claim 1 wherein the aqueous solution of gelatin iscooled to a temperature between about 0*C. and about -5*C.
 3. The methodof claim 1 wherein the gelatin powder is of substantially uniform sizeand is substantially free of water.
 4. The method of claim 1 wherein thefinely divided gelatin powder is added to the carrier liquid in anamount of form about 0.001 to about 5 percent by weight of the carrierliquid.